An inverter system radio frequency heating apparatus, for example, has a built-in transformer unit having a booster transformer packaged onto a substrate. A circuit of this transformer unit will be explained with reference to FIG. 9.
A commercial power source 51 is subjected to full wave rectification by a rectifier circuit 53 such as a diode bridge, is converted to a high frequency voltage by an inverter 55 and is then applied to a primary winding 59 of a booster transformer 57. Consequently, a high frequency high voltage of several kilo-volts develops in a secondary winding 61 of the booster transformer 57.
This high frequency high voltage is rectified by a voltage doubler rectifier circuit 67 including a capacitor 63 and a diode 65. Consequently, a high voltage is applied to a magnetron 69 as a microwave generator. A heater winding 71 of the booster transformer 57 is connected to a filament 73 of the magnetron 69 to heat the filament 73. The magnetron 69 oscillates the microwave by means of heating of the filament 73 and the application of the high voltage.
One of the booster transformers 57 for driving the magnetron has a construction in which the primary winding 59, the secondary winding 61 and the heater winding 71 are wound on one bobbin 75 as shown in FIG. 10, for example, and are juxtaposed on the same axis as those of U-shaped magnetic bodies 77 and 78. In such a booster transformer 57, a pin terminal connected to each winding is fitted and fixed into each terminal hole of the substrate on which the booster transformer 57 is to be packaged.
The booster transformer for driving the magnetron, having the construction described above is described in Japanese publication JP-A-10-27720, for example.
In the booster transformer for driving the magnetron of this kind, reduction of the size of apparatuses such as a heating cooking machine and mounting of components providing higher addition values with higher function of the apparatus have been required, and down-sizing of each part of the apparatus has been positively attempted. Among them, the boosting transformer is a component having particularly a large weight and a large capacity and reduction of its size has been required, in particular.
A bobbin 75 on which a primary winding 59 and a secondary winding 61 of the booster transformer are wound is produced by molding. When the shape of the bobbin 75 becomes complicated, a mold for molding becomes expensive and the cost of production increases. Particularly because the secondary winding 61 is formed into a plurality of layers such as three or more layers in some cases, the shape of the bobbin 75 gets complicated. When ribs 79 for dividing the winding area are simply omitted or the number of layers is decreased so as to simplify the shape of the bobbin, a line voltage increases to thereby induce corona discharge and service life of the transformer is drastically shortened.